(a) Field of the Invention
The present invention relates to a plastic guide post for magnetic tapes.
(b) Description of the Prior Art
The guide post which is used as a supply guide means for magnetic tapes accommodated in video cassettes, etc. must function so as to maintain images to be reproduced always clear while withstanding slide contact with magnetic tapes for a long time, and has conventionally been made of metals machined at high manufacturing cost. That is to say, the metal guide posts have been used widely for the merits of hardness, heat conduction property and antistatic property which are the factors for enhancing wear resistance and abrasion resistance as well as excellent precision on the surfaces thereof finished by machining, but have a serious defect that they require high machining cost.
In view of this defect, attempts have been repeated to make the guide posts of plastic materials for the purpose of reducing manufacturing cost through mass production. However, since the plastic materials are fundamentally low in heat dissipation thereof, it is undeniable that the plastic materials are low in resistance to longterm friction or abrasion especially when they are to be brought into slide contact with plastic films, and are low in surface precision as compared with machined metal guide posts. In other words, it is difficult to obtain surface roughness smaller than 1 micron by machining a plastic guide post into the same form as that of the metal guide post and, on the other hand, injection molding provides excellent surface roughness but remarkably degrades surface straightness, thereby often causing unsmooth running due to deformation or adhesion of the magnetic tape.
Further, long-term slide contact between the plastic guide post and the polyester film used as the base of the magnetic tape produces abrased powder and deformation of the magnetic tape, thereby degrading quality of reproduced images. Speaking concretely, the plastic guide post allows dropout error and production of horizontal stripes due to local demagnetization on reproduced images oftener than the metal guidepost and has a defect to make images very shabby.
Furthermore, since the plastic is originally an insulating material, it increases frictional torque due to electrical charging in a dry environment or at low temperatures, thereby making tape running unstable, imposing excessive load on the motor mounted on the deck or stopping tape running in extreme cases.
In order to correct those defects, the inventor et al proposed, in Japanese Preliminary Patent Publication No. Sho 60-28063 (Patent Publication No. Sho 63-49303), a guide post for magnetic tapes wherein tiny convexities and concavities are formed on the surface thereof for reducing friction, and electrical conductivity is imparted by using carbon fibers or the similar material as a conductive filler. However, subsequent researches have clarified that the convexities and concavities are formed by molding mostly on the edges of the end surfaces of the carbon fibers, become factors to produce flaws on the tape surface brought into contact with these convexities and concavities, and accelerate deterioration of reproduced images due to the tape damage.
Moreover, an attempt to obtain the characteristics similar to those described above by using super-fine powder of carbon black or the similar material also constitutes problems that is difficult to maintain surfaces of moldings in ideal shapes due to contamination of the mold by oil bleed at the molding stage since a liquid dispersion oil is used for mixing and kneading, and that tape damage is also caused due to adhesion of dust since the surface oil transfers to the tape.